My cat saw his tail hanging when he was on top of the computer and was spooked by it, then watching it like he was going to attack it, then was trying to play with it like the feather thingy I use with him everyday! I wish I had it on tape, it was too funny!

To a cat, play fighting is a good way to bond and learn a useful life skill. The cat is thinking "hey while we're at it let's get some practice in and also I love you why are you not patting me anymore they're just claws"

My cat doesn't do that when you rub his belly. He just opens his legs further and rolls to his back, watching you do it. He'll give a warning and then bite you if you scratch just above his tail, though.